The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating, but when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but is free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos Θ, where Θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of anti ferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer. With the ever increasing demand for improved data rate and data capacity, engineers and scientists have been under constant pressure to develop ever smaller magnetoresistive sensors. The various dimensions of a sensor scale together, so as the track width of a sensor decreases, the gap thickness and stripe height decrease accordingly.
With the drive for ever increased data rate and data density, researchers have focused their efforts on the development of current perpendicular to plane (CPP) magnetoresistive sensors such as CPP GMR sensors and tunnel valves. Such sensors, especially tunnel valves, have the potential to provide greatly increased sensor performance such as increased dR/R, decreased gap thickness (ie. bit length), and provide an improved ability to read signals from high coercivity media such as those used in perpendicular recording systems. Perpendicular recording systems are viewed as the future of magnetic recording, because of their ability to record much smaller bits of data than is possible using more traditional longitudinal recording systems.
CPP GMR sensors operate based on spin dependent scattering of electrons, similar to a more traditional current in plane (CIP) sensor. However, in a CPP sensor, current flows from the top to the bottom of the sensor in a direction perpendicular to the plane of the sensor. A tunnel valve, or tunnel junction sensor operates based on the spin dependent tunneling of electrons through a very thin, non-magnetic, electrically insulating barrier layer. A challenge that has prevented the commercialization of CPP GMR sensors, and tunnel valves, has been the shunting of current across the sensor. This is especially problematic for tunnel valves which rely on the high resistance of the barrier layer.
A method that has been used to construct magnetoresistive sensors involves depositing the sensor layers (eg. pinned layer spacer/barrier layer, free layer) as full film layers, and then forming a mask structure over the layers. The mask structure may include a non-photoreactive layer such as DURAMIDE®, and a photoresist layer formed over the DURAMIDE®. The photoresist layer is then patterned to have a width to define the sensor track width and stripe height (back edge). If a non-photoreactive intermediary layer is present, the pattern from the photoreactive layer has to be transferred to this non-photoreactive layer using a method such as reactive ion etching. With the mask in place a material removal process is performed to remove sensor material not covered by the mask. Usually two separate masking and milling processes are performed, one to define the stripe height and another to define the track width.
As a bi-product of the milling operation, material that has been removed during milling becomes re-deposited on the sides and back of the sensor. This re-deposited material has been referred to in the industry as “redep”. Such redep is undesirable in a CIP sensor because it increases parasitic resistance at the sides of the sensor and degrades free layer biasing. However, this redep is absolutely catastrophic in a CPP sensor such as a CPP GMR or a tunnel valve, because it allows sense current to be shunted through the redep, completely bypassing the active area of the sensor.
A process for removing the redep includes ion milling at a sharp, glancing angle to selectively remove material from the sides of the senor. However, this high angle ion milling causes damage to the sensor material such as by causing interlayer diffusion, resulting in a serious loss of sensor performance.
Therefore, there is a strong felt need for a method for manufacturing a magnetoresistive sensor that prevents all current shunting at the sides of the sensor. Such a method and structure would also preferably prevent damage to the sides of the sensor layer due to ion milling operations used to define the sensor.